X-ray Images of the Vela SNR

Craig Markwardt, University of Wisconsin at Madison

30 Oct 1996

Below are three images of the Vela supernova remnant, as taken from
the ROSAT All-Sky Survey. The intent is to look for spectrally "hard"
emission associated with the Vela pulsar wind nebula.

They represent, in order from left to right, a total count rate map
from the RASS (log stretch); a hardness ratio map of 0.5-0.9 to
0.1-0.4 keV channels (lin stretch); a hardness ratio map of 0.9-2.0 to
0.5-0.9 keV channels (lin stretch). Note that this is not the ideal
choice of bands for studying the high energy continuum because there
is a large amount of thermal emission from the supernova remnant shell
which dominates over any possible pulsar nebular contribution.
Unfortunately these are the bands available to the public (see
reference below). The images are in galactic coordinates (with
galactic north UP), each pixel being 40'x40'. The image intensity
scale is given by the color bar and the "CUTS=" parameter in each
image.

What can be seen is the Vela supernova remnant, which looks somewhat
like a human heart. The Puppis supernova remnant, which is more
distant, also appears as a smaller blob slightly to the right of
center in all images. The hardness ratio maps are meant to indicate
the spectral hardness or temperature of the emitting plasma. [Since
this is a ratio of count rate maps, the hardness maps will be weighted
by emission measure.]

In the central image (ratio of 0.5-0.9 to 0.1-0.4 keV) shows that the
SNR is not uniform: there is harder emission at the bottom part of the
remnant. This is probably because there is a lack of soft thermal
shell emission from this part of the remnant compared to the other
parts. [One suggestion is that the exterior medium is less dense on
this side of the remnant, and so less shock heating takes place there
as the remnant expands.]

The right-hand image (ratio of 0.9-2.0 to 0.5-0.9 keV) shows that the
remnant is much more uniform, at least azimuthally. The center of the
remnant shows as a yellow blob, indicating slightly harder emission
than other parts of the remnant. [Puppis A, is much harder because of
its distance and interstellar absorption, and appears as a red blob to
the right.] The very fringes of the remnant appear to be hard, but
have such very poor statistics that they are probably unreliable.

We might claim that the yellow blob is an extended plerion at the
center of the Vela supernova remnant. We must consider, however, that
the image has an angular resolution of 2 degrees (the field of view of
ROSAT). The central blob has a diameter of approximately 2 degrees as
well. This means that the central blob we observe could actually be a
point-source like object, such as the compact nebula of the Vela
pulsar, which is indeed very bright, small (~2 arcmin diameter), and
spectrally hard. A proper analysis would need to do several things:
use higher-energy channels, which are free from problems of shell
thermal emission; exclude known point sources which are in the field
before rebinning to a coarse scale. This would include the compact
nebula and Puppis A if possible.